The air pump with an air chamber of a large cross-sectional area is capable of inflating an inflatable object rapidly. However, the user of such air pump must make a greater physical effort to complete the inflating process. On the other hand, the pumping job can be made easier by reducing the cross-sectional area of the air chamber of the air pump. However, the pumping job can not be done rapidly with the air pump having an air chamber of a relatively small cross-sectional area. Generally speaking, the conventional air pumps are incapable of inflating an inflatable body rapidly at such time when air pressure level in the inflatable body is high.